Add "readOnly"-endpoint parameter to repo-config endpoint

The repo-config endpoint allows plugins to add new config sections to
the repository configuration. So far, each plugin had to determine itself,
whether the options were changeable by the logged in user. Thus, each
plugin that wanted to disable form fields, if the user was not the owner
of the repo, had to perform the same REST-request, which led to an
unnecessarily high number of requests.

This change adds a parameter to the repo-config endpoint, which forwards
the boolean _readOnly variable to the plugin to be used to decide, whether
an option should be enabled to the logged in user.

Change-Id: Id1d7db2184d84d4ded157bed9a0492f516eeb53e

polygerrit-ui/app/elements/admin/gr-repo/gr-repo.html

@@ -353,6 +353,7 @@ limitations under the License.
           </fieldset>
           <gr-endpoint-decorator name="repo-config">
             <gr-endpoint-param name="repoName" value="[[repo]]"></gr-endpoint-param>
+            <gr-endpoint-param name="readOnly" value="[[_readOnly]]"></gr-endpoint-param>
           </gr-endpoint-decorator>
         </div>
       </div>